Cited2 [CBP/p300-interacting transactivators with glutamic acid (E) and aspartic acid (D)-rich tail 2] is one of the founding members of a new family of transcriptional activators. My laboratory cloned Cited2 and showed that Cited2 is induced by many biological stimuli such as cytokines, serum and lipopolysaccharide (LPS) in different cell types and it is a transforming gene when overexpressed. Cited2 acts as a transcriptional modulator by interacting with DNA binding proteins (Tfap2, Smad2/3, Lhx2, PPAR, and PPAR) to regulate the transcription of their corresponding target genes. It also functions as a negative regulator of HIF-1 by competing with HIF-1 in binding to the first cysteine-histidine-rich (CH1) region of CBP/p300 to inhibit HIF-1 mediated signaling. Mice lacking Cited2 die at late gestation with numerous developmental defects. Recently, we observed pulmonary immaturity in E17.5 to E18.5 Cited2-/- lungs, which display significantly reduced alveolar space due to an arrest in the type I and type II alveolar cell differentiation and decreased number of apoptotic alveolar cells. Consistent with the phenotype, genes associated with alveolar cell differentiation were differentially expressed between wild-type and Cited2-/- embryonic lungs. These results suggest that Cited2 may be a potential candidate for respiratory distress syndrome. We further demonstrated that expression of C/EBP, one of the key regulators of airway epithelial maturation, was significantly decreased in Cited2-/- embryonic lungs. Interestingly, many of the phenotypes in Cited2-null embryonic lungs were similar to those in mice bearing lung-specific deletion of C/EBP and deletion in the CH1 domain of CBP. In transient transfection assays, Tfap2c functioned as a transcriptional repressor of the C/EBP gene promoter in HeLa and A549 cells and this repressive effect was abolished by co-transfection of Cited2. Thus, we hypothesize that Cited2-Tfap2c-CBP complex controls lung maturation by regulating C/EBP expression. To test this hypothesis, we will (1) study the regulatory mechanism of Cited2 on C/EBP expression, (2) use genetic approaches to show that C/EBP is responsible for the pulmonary immaturity in Cited2- deficient embryonic lung during development, and (3) study the molecular mechanism of alveolar cell survival and differentiation arrest in Cited2-deficient embryonic lung. The accomplishment of the study will not only elucidate molecular mechanisms of lung development but also provide new clues to the molecular pathogenesis of respiratory distress syndrome and help identify targets for specific therapies.